Such sexual symptoms have long been known side effects of the popular Prozac class of antidepressants, but a growing body of research suggests that they are far more common than previously thought, perhaps affecting half or more of patients….

Current warnings on the labels of selective serotonin reuptake inhibitors, or SSRIs, cite early studies in which the prevalence of sexual side effects was lower: 4 percent for Prozac, for example, and ranging from 0 to 28 percent for Paxil.

But more recent studies, in which patients were more likely to be asked about specific sexual side effects and thus more likely to report them, suggest that the ballpark range of those affected by SSRIs is between 30 percent and 50 percent, said researchers including Dr. Richard Balon, a psychiatry professor at Wayne State University who studies the symptoms.

That would translate into millions of affected sex lives among the estimated 1 in 8 American adults who have tried these antidepressants in the past decade or so. Some studies have found the range still higher.

Why the rising costs and flattening benefits? Among other things, it’s becoming clear, as the wide use of these drugs runs through time, that many drugs prove less effective and more troublesome when prescribed to sick people (many of whom have other health problems and take other drugs) than when used in clinical trials, which usually take care to use patients with fewer problems. It doesn’t help that drug companies often fail to report or publish their less flattering results — and that they didn’t investigate the sexual side-effects more aggressively during the trials.

In this case the differences between side effects in trials and in real life is startling, both for the scale of the difference and, of course, for the high-impact nature of sexual side-effects. As Aline Zoldbrod, a Lexington psychologist and sex therapist quoted in the Globe article notes:

“This is such an upsetting issue. There are people for whom SSRIs are really life-saving, I think, but the idea that someone would have to choose between getting out of the darkness of depression and having a good sex life is horrible.”